Magneto-inductive, flow-measuring devices use the principle of electrodynamic induction for volumetric flow measurement: Charge carriers of the medium moved perpendicularly to a magnetic field induce a voltage in measuring electrodes arranged likewise essentially perpendicularly to the flow direction of the medium. This voltage induced in the measuring electrodes is proportional to the flow velocity of the medium averaged over the cross section of the tube; it is, thus proportional to the volume flow.
The measuring electrodes are coupled with the medium either galvanically or capacitively. If the measuring electrodes come in contact with the medium, then a coating forms on the surface of the measuring electrodes over the course of time. A result of this coating formation is a malfunctioning of the flow measuring device. If the coating is composed of a non-conducting material, then the flow measuring device no longer delivers any measured values at all.
Although, in what follows, coating formation on the measuring electrode will always be described as the cause of the malfunctioning, or non-functioning, of the flow measuring device, the invention is generally applicable also for the detection of other malfunctions, which can arise in the case of a magneto-inductive, flow-measuring device.
For removing undesired coatings of conductive material from the measuring electrodes, EP 0 337 292 proposes cleaning the measuring electrodes in predetermined intervals of time by application of an electric DC or AC voltage. In spite of the considerable advantages that an automatic cleaning of the measuring electrodes provides compared with a manually executed cleaning, the known automatic cleaning of the measuring electrodes also has disadvantages: First of all, it can not be universally applied for coatings of all materials—it functions only for removing conductive coatings. Moreover, the automatic cleaning is done as preventive maintenance in predetermined intervals of time; it is, thus, not assured that the cleaning will occur at a point in time when it is really needed.
Preventive cleaning of the measuring electrodes is, for a number of reasons, disturbing and undesired: Thus, for a certain period of time following the cleaning procedure, no volumetric flow measuring is possible, since the measurement voltage must first rise anew on the measuring electrodes. Furthermore, the electrical current supply for cleaning of the measuring electrodes occurs over a rigidly predetermined period of time, since the degree of the coating formation is largely unknown at the point in time of the cleaning. It is, thus, left to chance or to the related experience of the operating personnel, whether, following accomplishment of the automatic cleaning, the desired optimum state of the measuring electrodes has actually been reached. In the normal case, it can be assumed that, following automatic cleaning, either there is still coating on the measuring electrode, or the measuring electrode has been damaged because the cleaning voltage was applied for a longer period of time than necessary.
Still a greater problem are the interruptions of the measurements, when the measuring electrodes must be freed of non-conductive coatings as a part of preventive maintenance. The downtimes of the flow measuring device are yet greater in this case, since removal of non-conductive coatings can only be done by mechanical methods; i.e. the measuring device must be uninstalled and the measuring electrodes cleaned manually.
Known from EP 1 108 988 A1 is a solution concerning how a coating formation can be detected accurately and automatically, in order then, as required, for such to be removed from the measuring electrode. To this end, a defined test signal is sent to the measuring electrode; on the basis of the signal in response to the defined test signal and/or on the basis of a reference quantity determined from the signal in response to the defined test signal, it is determined whether the measuring electrode is delivering correct measured values. The signal in response to the defined test signal and/or the reference quantity determined from the signal in response to the defined test signal is referred to, in the following, for the sake of simplicity, as the actual value. By the method of the invention, a gradually approaching malfunctioning of the flow measuring device is detected early, so that it can then be opposed with targeted countermeasures. Especially, it is proposed in this Offenlegungsschrift to compare each actual value with a predetermined, desired value and to display, output and/or store an indication of malfunction, when the actual value deviates from the desired value.
The known solution provides the desired success in many respects. Measurement errors and erroneous interpretations can, however, arise, when the relaxation time of the signal in response to the test signal sent to the measuring electrode exceeds the duration of a measuring period. Due to the switching of the magnetic field after a measuring period, the situation can arise in which the control/evaluation unit signals a reaching of the desired value; yet, in fact, the supposed actual value is not an indication of the exceeding of a tolerable coating formation, but, instead, results from the superposition of different voltage values on the measuring electrode.